Dissociative Attachment Rate Research Articles

Investigation on plasma enhanced decomposition of ammonium dinitramide (ADN) based propellant with optical diagnosis

Plasma-enhanced decomposition is a promising method to achieve reliable ignition and better thruster performance with an ADN-based propellant. High-speed imaging and optical emission spectroscopy are used to characterize the discharge plasma produced from ADN-based propellant vapor and argon carrier gas within parallel planes. Discharge regimes are independently controlled by varying working parameters. The transition from abnormal glow discharge (AGD) to filamentary discharge (FD) is identified from voltage-current characteristics and sudden changes in excitation electron temperature (Te-exc) and electron density (Ne) under a pressure of 0.2–10 kPa. Instabilities develop because there are more freedom degrees and higher collision probabilities after ADN-based propellant vapor is added. The product of Te-exc and Ne shows a higher energy transfer efficiency from input power to vapor in the FD regime. Water molecules increase the net dissociative attachment rate and effectively quench Ne. Although preheated vapor (as the discharge medium) has a much lower Ne, more radicals appear such as OH, NH, CH, CN, N2, N2+, and C2, which promote chemical interactions in an electric field and increase the probability of successful ignition. This can be attributed to thermalization due to an increase in the translational kinetic energy and rotational excitation, which has an important impact on chemical reaction kinetics. Therefore, preheating is verified as critical for improving the ignition and performance of a plasma-assisted ADN-based thruster.

Calculation of rate constants for dissociative attachment of low-energy electrons to hydrogen halides HCl, HBr, and HI and their deuterated analogs

Calculations of rate constants for the process of dissociative attachment of low-energy electrons to hydrogen halides HCl, HBr, and HI and for the reverse process of associative detachment based on the nonlocal resonance model are reported. The calculated data are of importance for the modeling of plasma processes, environmental chemistry, etc. The calculated dissociative attachment rate constants are found to be in good agreement with existing experimental data. It is shown that at low temperatures the rate constants are very sensitive to small changes of the parameters of the nonlocal resonance model used for the calculation of the rate constants and represent a severe test of the theory. The isotopic effect and its dependence on the temperature is also discussed. The calculations of rate constants for the reverse process of associative detachment are also reported and discussed.

Dissociative attachment of low-energy electrons to state-selected diatomic molecules

The dissociative attachment process in low-energy collisions [Na{sub 2}({ital v}{sup {double_prime}},{ital j}{sup {double_prime}})+{ital e}{sup {minus}}{r_arrow} Na + Na{sup {minus}}, with electron energies {lt}1.0 eV] is studied as a function of vibrational excitation over a broad range of excited levels. We employ a crossed electron-molecule beams arrangement and two optical methods for preparing vibrationally excited states: Franck-Condon pumping (FCP) as well as the efficient and highly state-specific technique of stimulated Raman scattering with adiabatic passage (STIRAP). Both methods are capable of preparing vibrational levels with energies up to about 60{percent} of the Na{sub 2}{ital X}{sup 1}{Sigma}{sub {ital g}}{sup +} binding energy ({ital v}{sup {double_prime}}{le}30). Analysis of the FCP data shows an increase of more than three orders of magnitude in the state-dependent dissociative attachment rate as a function of the vibrational level up to {ital v}{sup {double_prime}}=12. For {ital v}{sup {double_prime}}{approx_gt}12, which is close to the exoergic threshold, no further increase with {ital v}{sup {double_prime}} is observed. The dissociative attachment rates measured for single rovibronic states using the STIRAP technique confirm the validity of the conclusions drawn from our analysis of the FCP measurements. The potential curve of the Na{sub 2}{sup {minus}}{ital A}{sup 2}{Sigma}{sub {ital g}}{sup +} state, which is essentialmore » for the dissociative attachment process, has been determined from variational valence shell multireference configuration interaction calculations in its bound region but by imposing Feshbach projection in its resonance region. The crossing of this potential with that of the Na{sub 2}({ital X}) state is found between {ital v}{sup {double_prime}}=11 and {ital v}{sup {double_prime}}=12, in full agreement with the conclusions from analysis of the experimental data. (Abstract Truncated)« less

The chemistry of a CCl2F2 radio frequency discharge

A systematic study of the chemistry of stable molecules and radicals in a low pressure CCl2F2 radio frequency discharge for dry Si etching has been performed. Various particle densities have been measured and modeled. The electron density, needed as an input parameter to model the CCl2F2 dissociation, is measured by a microwave cavity method. The densities of stable molecules, like CClF3, CF4, 1,2-C2Cl2F4 and the etch product SiF4, are measured by Fourier transform absorption spectroscopy. The density of the CF2 radical is measured by means of absorption spectroscopy with a tunable diode laser. Its density is in the order of 1019 m−3. All density measurements are presented as a function of various plasma parameters. Moreover, optical emission intensities of Cl and F have been recorded as a function of plasma parameters. It appears that the feed gas (CCl2F2) is substantially dissociated (about 70%–90%) in the discharge. Based on the obtained data the dissociation rates of several molecules have been estimated. The measured total dissociation rate of CCl2F2 is 8×10−15 m3 s−1. For this molecule the dissociation rate is substantially higher than the dissociative attachment rate (10−15 m3 s−1). The dissociation rate for CClF3 is 2×10−15 m3 s−1 and for CF4 about 3×10−16 m3 s−1. The total dissociation rate of C2Cl2F4 is higher than 5 ×10−15 m3 s−1, of which 2.5±0.5 × 10−15 m3 s−1 is due to dissociative attachment. Furthermore it has been found that the presence of a silicon wafer strongly affects the plasma chemistry. Optical emission measurements show that the densities of halogen radicals are significantly depleted in presence of Si. Moreover, the densities of several halocarbon molecules display a negative correlation with the density of the etch product SiF4.

Vibrational dependence of negative-ion formation by dissociative attachment of low-energy electrons.

The vibrational dependence of dissociative attachment in low-energy ${\mathrm{Na}}_{2}$(v'')+${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ collisions (${\mathit{E}}_{\mathit{e}\mathit{l}}$1.0 eV) is studied with a crossed-beam apparatus. The ${\mathrm{Na}}_{2}$ vibrational distribution is controlled through Franck-Condon pumping. A range of vibrational levels with energies up to approximately 60% of the molecular binding energy is covered. We observe a three orders of magnitude increase in the dissociative attachment rate for low lying and intermediate vibrational levels v''. Further analysis shows a strikingly abrupt leveling off above v''\ensuremath{\approxeq}12.

Isotope effect and electron-temperature dependence in volume H- and D- ion sources.

We present the results of a numerical study of isotope effects in volume negative-ion sources and compare them with experimental results. We have studied both hydrogen and deuterium discharges over a range of discharge current. For basic plasma parameters, the model reproduces the behavior measured in the discharge region of a tandem multicusp source. The theory predicts a local maximum in negative-ion production at an optimal electron temperature ${\mathit{T}}_{\mathit{e}}^{\mathrm{opt}}$\ensuremath{\approxeq}0.6--0.8 eV. This prediction is supported by our analysis of ${\mathrm{D}}^{\mathrm{\ensuremath{-}}}$ and ${\mathrm{H}}^{\mathrm{\ensuremath{-}}}$ densities measured in a hybrid multicusp source and in the extraction region of a tandem source. We show how the local maximum is related to the temperature dependence of the dissociative attachment rate coefficients. The most significant isotope effect suggested by this modeling effort is a stronger cooling of the vibrational distribution in deuterium due to vibration-translation reactions between molecules and atoms. This vibrational cooling reduces negative-ion production in high-power deuterium discharges below that in equivalent hydrogen discharges.

Electron scattering and dissociative attachment by SF6 and its electrical-discharge by-products

Discrete electron-molecule processes relevant to SF6 etching plasmas are examined. Absolute, total scattering cross sections for 0.2–12-eV electrons on SF6, SO2, SOF2, SO2F2, SOF4, and SF4, as well as cross sections for negative-ion formation by attachment of electrons, have been measured. These are used to calculate dissociative-attachment rate coefficients as a function ofE/N for SF6 by-products in SF6.

Time-resolved electron density measurements in an ArF excimer laser discharge

The electron density in an ArF excimer laser discharge has been measured by means of a time-resolved optical interferometric method. By comparing the behavior of electron density in the absence of F 2 gas with that in the presence of F 2 gas, the dissociative attachment rate is estimated from graphical calculation.

Electron dissociative attachment to nitrogen difluoride radicals

Measurements of discharge current in an electron beam sustained-discharge device have been used to determine the dissociative attachment rate constant of electrons to NF{sub 2} radicals. This was done by measuring the relative rate of electron attachment to NF{sub 2} compared to dilute molecular chlorine in a nitrogen buffer in separate experiments in the same device. A comparison of the steady-state current measured for these two different mixtures at identical total pressures and applied electric fields provides a relative measurement for the rate of disappearance of electrons. Since the attachment rate constant with molecular chlorine is well-known, rate constants for the fluoride radical could be determined. Values near 1.5 {times} 10{sup {minus}9} cm{sup 3}/s for average electron energies near 1 eV were obtained.

Dissociative attachment in optically pumped lithium molecules.

Experimental observations of dissociative attachment in lithium are described. Using laser excitation of a supersonic lithium beam a population of metastable vibrationally excited lithium molecules has been created. A cold-electron population (0.13 eV) has been created by laser photoionization of lithium atoms, and the attachment of these electrons has been studied as a function of molecular state. The dissociative attachment rate constant does not vary significantly for ${\mathrm{Li}}_{2}$(X) between v\ensuremath{\sim}10 and v\ensuremath{\sim}13, states which lie above the attachment energy threshold, nor is there significant rotational variation between J\ensuremath{\sim}0 and J\ensuremath{\sim}20. The rate constant for dissociative attachment to these states for thermal electrons at \ensuremath{\sim}0.05 eV has been measured to be (2\ifmmode\pm\else\textpm\fi{}1)\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}8}$ ${\mathrm{cm}}^{3}$ sec $^{\mathrm{\ensuremath{-}}1}$.

Generation of negative ions in tandem high-density hydrogen discharges

An optimized tandem two-chamber negative-ion source system is discussed. In the first chamber high-energy (E>20 eV) electron collisions provide for H2 vibrational excitation, while in the second chamber negative ions are formed by dissociative attachment. The gas density, electron density, and system scale length are varied as independent parameters. The extracted negative ion current density passes through a maximum as electron and gas densities are varied. This maximum scales inversely with system scale length R. The optimum extracted current densities occur for electron densities nR=1013 electrons cm−2 and gas densities N2R in the range 1014–1015 molecules cm−2. The extracted current densities are sensitive to the atomic concentration in the discharge. The atomic concentration is parametrized by the wall recombination coefficient γ and scale length R. As γ ranges from 0.1 to 1.0 and for system scale lengths of 1 cm, extracted current densities range from 8.0 to 80 mA cm−2. The relative negative-ion yields from single-chamber and tandem two-chamber systems are compared. Estimates are made for the rates of polar dissociation of H2 molecules and H+3 ions, and these rates are compared with the dissociative attachment rates.

Ground-state dissociative attachment rates in HCl and Cl2

Ground-state dissociative attachment rates in HCl and Cl2 are obtained in the electron energy ranges from 0.7 to 5 eV. The results are compared with previous cross-section data convoluted with Boltzmann-code calculations for the electron energy distribution.

Electron dissociative attachment rate constants for F2 and NF3 at 300 and 500 °K

Electron attachment to F2 and NF3 has been studied in an electron-beam-controlled gas-discharge apparatus over a range of E/P (2–10 kV/cm atm). These experiments were performed in gas mixtures containing small amounts of the halide molecules (≲1%) in an atmosphere of N2 which was included to control the average electron energy. We obtained values for the rate constants for dissociative attachment to F2 and NF3 as a function of mixture temperature at 300 and 500 °K and applied electric field. These results compare favorably with the rate constants deduced from the absolute cross section for these compounds reported by Chantry.

Dissociative attachment of electrons to Br2 and HBr

Electron attachment to Br2 and HBr has been studied over a range of E/P values using a high-pressure electron-beam-sustained gas-discharge apparatus. Experiments were performed in gas mixtures containing the attaching species and a large excess (⩾99%) of N2 which was included to control the electron temperature. Values of the dissociative attachment rate constants at an average electron energy near 1.0 eV for Br2 and HBr were found to be 1.0×10−10 and 8×10−10 cm3/sec, respectively. Using the known electron energy dependence of the dissociative attachment cross section, the magnitude of the dissociative attachment cross section for HBr was estimated.

Dissociation of fluorine by electron impact

Using a previously developed F-atom diagnostic technique, the efficiency of F2 dissociation by an electron beam or an electron-beam-sustained discharge was systematically studied. By monitoring the transient concentration of ions generated during the electron-beam-sustained discharge, we have determined the F−+F+2 ion-ion recombination and F2 dissociative attachment rate constants at various E/P conditions. The dissociative attachment rate constant of F2 at an average electron energy of 1 eV was found to be about (2.3±0.3) ×10−9 cm3/molecule sec−1 and increased with decreasing electron energy. The F−+F+2→3F recombination rate constant at an E/P of 18 kV/cm/atm was found to be about (1.5±0.2) ×10−8 cm3/molecule sec−1 and also increases with decreasing E/P values. The sustained-discharge enhancement of F-atom production from these experiments is F2 is small, ∼1.7.

Dissociative attachment of electrons to F2

By measuring the equilibrated sustainer current density in F2/N2 mixtures under controlled discharge conditions, we have determined the F2 dissociative attachment rate constant at various E/P conditions. The F2+e→F∓F rate constant at an average electron energy of 1.0 eV was found to be 2.3±0.3×10−9 cm3/molecule/sec and increased with decreasing electron energy.

Measurement of thermal electron dissociative attachment rate constants for halogen gases using a flowing afterglow technique

A flowing afterglow apparatus was constructed and used to measure thermal electron attachment rate constants in the halogen gases fluorine, chlorine, and bromine. The operation of the afterglow system and the mathematical models applied in treating the data were tested by measuring the thermal attachment rate constant for electrons in sulfur hexafluoride. The average value obtained for this rate constant is 4.2±1.1×10−8 cm3 molecule−1 sec−1 when a microwave discharge was used as the electron source and 3.6±1.8×10−8 cm3 molecule−1 sec−1 when a filament was used as the electron source. The average electron temperature was estimated to be approximately 600 °K for the microwave discharge source and approximately 350 °K for the filament source. A charge transfer reaction between sulfur hexafluoride and the ion O2− was also investigated in the present study to further assess the operation of the flowing afterglow apparatus in the microwave discharge source configuration. The average rate constant obtained for this reaction is 3.7±0.4×10−11 cm3 molecule−1 sec−1 for an estimated ion temperature of 300–325 °K. The average rate constants obtained for the dissociative attachment of electrons in fluorine are 4.6±1.2×10−9 cm3 molecule−1 sec−1 for an electron temperature of approximately 600°K and 3.1±1.2×10−9 cm3 molecule−1 sec−1 for an electron temperature of approximately 350 °K. The average rate constants obtained for dissociative electron attachment in chlorine and bromine are 3.7±1.7×10−9 and 1.0±0.9×10−11 cm3 molecule−1 sec−1, respectively, for an electron temperature of approximately 350 °K. The rate constant for the three-body attachment reaction Br−+Br2+Ar to form Br3− was also measured and found to be 1.9±0.5×10−28 cm6 molecule−2 sec−1.

Attachment instability in an externally ionized discharge

A new type of plasma instability is described in an electron-beam ionized discharge. This instability occurs in the form of current oscillations in gas mixtures in which the dissociative attachment rate increases strongly with electric field. It has been observed experimentally in He : H2O 74 : 2 and in He : CO2 1 : 1 and 9 : 1 mixtures. A theoretical analysis which describes the onset of instability is presented, and a physical explanation is given. The region of instability in parameter space of ionization source function S and electric field E is presented for certain gas mixtures. The transient phenomena in this type of discharge appear to be controlled by electron attachment, and a theoretical explanation of the observed current overshoot has been provided.

An electron attachment plasma instability

A new type of plasma instability is described in an electron-beam ionized discharge. This instability occurs in gas mixtures in which the dissociative attachment rate increases strongly with electric field. It has been observed experimentally in He:H2O 740:20 and in He:CO2 1:1 and 9:1 mixtures. The values of the ionization source function S and of the electric field E at which the wave instability occurs are predicted using a Fourier analysis of the linearized kinetic rate equations. This yields the condition for instability as S < (1/α)[Eδβ/δE) − β]2, where α is the electron-ion recombination coefficient and β is the electron neutral attachment rate.